An alternative view is that the olivocerebellar system is directly involved in motor control (note that this does not preclude a role in motor learning as well) and, in particular, helps in the timing of motor commands. This view follows from the types of motor deficits observed in cerebellar damage and accounts for the special properties of the inferior olivary nucleus mentioned earlier: namely, that it can generate rhythmic, synchronous complex spike discharges across populations of Purkinje cells. These complex spikes would then produce synchronized inhibitory postsynaptic currents (IPSPs) on cerebellar nuclear neurons as a result of the convergence present in the Purkinje cell axon to the cerebellar nuclear projection. Because of the membrane properties of cerebellar nuclear neurons, these synchronized IPSPs could have a qualitatively different effect on nuclear cell firing than would the IPSPs caused by simple spikes, which are more numerous but largely asynchronous. Specifically, they could trigger large precisely timed changes in the nuclear cell activity that would then be transmitted to other motor systems as a gating signal. In fact, voluntary movements appear to be composed of a series of periodic accelerations that reflect a central oscillatory process. However, determining whether the olivocerebellar system helps time motor commands requires further evidence.
Motor Control by the Basal Ganglia
The basal ganglia are deep nuclei in the cerebrum. Like the cerebellum, a major function of the basal ganglia is to regulate motor activity and we will focus on that role in this section. However, it is worth noting that they, like the cerebellum, also contribute to affective and cognitive functions. To understand basal ganglia function in motor control, the following discussion is organized around two major themes: (1) the connections between basal ganglia and cortex form loops through which activity flows and (2) there are two functionally distinct pathways through the basal ganglia, the direct and indirect pathways.
Organization of the Basal Ganglia and Related Nuclei
The basal ganglia include the caudate nucleus, the putamen, and the globus pallidus (
Fig. 9.24 ). The term striatum, derived from the striated appearance of these
CHAPTER 9 Organization of Motor Function •Fig. 9.24 Components of Basal Ganglia and Other Closely Associated Brain Regions. Themaincomponentsofthebasalgangliaarethecaudatenucleus,putamen,globuspallidus,andsubstantianigraparsreticulata.Majorportionsofthebasalgangliaconnectwithmotorareasinthefrontalcortex,viatheventralanteriorandventrallateralthalamicnuclei,andwiththesuperiorcolliculus.Inputfromthesubstantianigraparscompactaiscriticalfornormalbasalgangliafunction.GPe,externalsegmentofglobuspallidus;GPi,internalsegmentofglobuspallidus.
nuclei, refers only to the caudate nucleus and putamen. The striations are produced by the fiber bundles formed by the anterior limb of the internal capsule as it separates the caudate nucleus and putamen. The globus pallidus typically has two parts: an external segment and an internal segment. The combination of putamen and globus pallidus is often referred to as the lentiform nucleus.
Associated with the basal ganglia are several thalamic nuclei. These include the ventral anterior (VA) and ventral lateral (VL) nuclei and several components of the intralaminar complex. Other associated nuclei are the subthalamic nucleus of the diencephalon and the substantia nigra of the midbrain (see
Fig. 9.24 ). The substantia nigra (“black substance”) derives its name from its content of melanin pigment. Many of the neurons in the pars compacta of this nucleus contain melanin, a byproduct of dopamine synthesis. The other subdivision of the substantia nigra is the pars reticulata. This structure can be regarded as an extension of the internal segment of the globus pallidus because these nuclei have an identical origin and analogous connections.
Connections and Operation of the Basal Ganglia